FIG. 1 illustrates an example mobile communications network 10. A radio access network (RAN) 12 is coupled to one or more core networks 14, which in turn, is(are) coupled to one or more external networks 16, like the Internet, the PSTN, ISDN, etc. The radio access network 12 includes, for example, one or more radio network controllers (RNCs) 18 that may communicate signaling and/or traffic to each other. Each RNC 18 controls one or more radio base stations (BSs) 20. Base stations may also be referred to as Node B's or access points. Each base station 20 transmits information over an “air” or wireless interface in one or more corresponding coverage areas called cells via a variety of downlink radio channels. Each base station 20 also receives uplink communications over the air interface from user equipments (UEs) (22) in or near that base station's cell(s) via one or more uplink radio channels. UEs are often referred to as mobile stations, mobile radios, and mobile terminals and include, for example, cell phones, PDAs, laptop computers, and other devices for wireless communication.
In mobile radio communications, a variety of different type channels may be used to convey different types of information. For example, channels may be defined as control/signaling channels or traffic channels, or they may be characterized as dedicated or common/shared channels. In third generation, Wideband-code division multiple access (WCDMA) cellular communications systems, the physical channels are classified in many ways. Examples of different type radio channels are conceptually represented in FIG. 1 including: one or more dedicated data channels like a Dedicated Channel (DCH), Enhanced Dedicated Channel (E-DCH), or Dedicated Physical Data Channel (DPDCH), etc., one or more dedicated control channels like a Dedicated Physical Control Channel (DPCCH), one or more shared data channels like a Random Access Channel (RACH), and a high speed shared channels like a High Speed-Downlink Shared Channel (HS-DSCH), an Enhanced-Dedicated Physical Control Channel (E-DPCCH), or a High Speed-Dedicated Physical Control Channel (HS-DPCCH).
From one perspective, the 3GPP UMTS FDD standard has evolved in three steps when it comes to substantial changes related to the physical layer (L1) processing. First, in Release R99, the basics of WCDMA were established, and the dedicated channel (DCH) was proposed as the transport channel for both circuit-switched and packet-switched data. The R99 physical channel for the DCH is called a dedicated physical channel (DPCH) and includes both a dedicated physical data channel (DPDCH) and a dedicated physical control channel (DPCCH). The symbols on the uplink (UL) DPDCH are direct-sequence spread by a channelization code cch with a spreading factor (SF) between 4 and 256, depending on the size of the data payload. The bits of a spreading code are called chips. If Tb represents the period of one data bit and Tc represents the period of one chip, the chip rate, 1/Tc, is often used to characterize a spread spectrum transmission system like WCDMA. The spreading factor (sometimes called processing gain) is defined as the ratio of the information bit duration over the chip duration: SF=Tb/Tc. In short, the spreading factor represents the number of chips used to spread one data bit. In general, a higher the spreading factor (the more chips in the spreading code), the lower the bandwidth and data rate. A higher spreading factor results in a higher signal-to-interference ratio of the despread signal in the receiver given a certain power of the spread signal. Conversely, higher data rates (high bandwidth requirements) use a lower spreading factor. Even higher bandwidth/data rate can be obtained by spreading data to more than one spreading code. A higher spreading factor also means that more spreading codes can be allocated on the same frequency channel.
A receiver for processing direct sequence spread spectrum signals that are received over a multi-path fading channel is illustrated in FIG. 2. The structure up to and including the Rake demodulator 46 is usually referred to as a Rake receiver. The Rake receiver is a diversity combiner exploiting the time diversity provided by the multi-path radio channel. The diversity branches are referred to as Rake fingers and are indicated as 41a, 41b, . . . 41n. Each Rake finger works on a differently delayed version of a composite (multiple UE) baseband signal. Each Rake finger's corresponding delay and downsample block 42a, 42b, . . . 42n first delays the data, mathematically represented as zτ−T, and then downsamples the data to the chip-rate, i.e., one times oversampling (1×OS). The product of a scrambling code cscr corresponding to a particular UE and a channelization code cch corresponding to a particular traffic channel is multiplied in corresponding multiplier 43a, 43b, . . . , 43n with the delayed and downsampled baseband composite signal to generate a complex-valued, UE-specific symbol stream at each Rake finger. The delayed composite signal is descrambled and despread to a UE-specific, complex-valued symbol stream at the rate that the composite signal is received. As the last stage of the Rake finger despreader, each rake finger's UE-specific, complex-valued symbol stream is integrated and “dumped” (44a, 44b, . . . 44n) over SF chips to produce a UE-specific, per-rake-finger user data symbol stream. The “integrate and dump over SF chips” operation corresponds to accumulating a number of consecutive symbols equal to the spreading factor and then outputting the accumulated sum. Thereafter, the accumulator is reset, and the next SF consecutive symbols are accumulated, etc. Determining which SF symbols to integrate and dump is a synchronization task for the Rake receiver and is not necessary for an understanding of the technology described here. The symbols generated from each of the rake fingers are channel-compensated in a Rake demodulator 46 by multiplying, at a corresponding multiplier 52a, 52b, . . . 52n, each user data symbol with a corresponding channel estimate ha(t), hb(t), . . . , hn(t). The channel-compensated symbols from all the Rake fingers are combined into one symbol stream received from a UE in a Rake combiner 53. The output of the Rake combiner 53 includes “soft” symbol values used in a decoder 58 to generate the actual data received from the UE.
An example of air-timing relations between various physical channels is illustrated in FIG. 3. The DPDCH is divided into 10 msec radio frames. Each radio frame is associated with 15 slots on the DPCCH. Each slot contains includes a 10-bit code word, with typically six pilot bits, two Transport Format Combination Indicator (TFCI) bits, and two transmit power control (TPC) bits. Significantly, the transport format used for the DPDCH is not explicitly available until the whole frame has been received and demodulated. Each TFCI is 30 bits long, but each slot only provides two of the 30 TFCI bits. As a result, all 15 slots in the frame must be decoded in order to obtain and then concatenate together all 30 TFCI bits.
When high speed data packet access (HSDPA) was introduced in the downlink, a new physical control channel was created in the uplink (UL)the HS-DPCCH. After that, enhanced uplink (E-UL) was standardized for the uplink to decrease user data latency, increase user data peak rates, and increase the air interface capacity. Another objective of enhanced uplink is to permit more users to transmit at high peak rates. A new transport channel, E-DCH, was introduced with release R6 for the implementation of E-UL. The physical channel for the E-DCH remains the DPCH which can either be a carrier of a pure E-DCH, mixed E-DCH and DCH, or a pure DCH. To support high peak rates, spreading factors down to 2 and “multi-code ” are used. Multi-code means that more then one spreading code is assigned to spread and transmit user data. In addition to the introduction of very low spreading factors and multi-code, a new physical control channel, E-DPCCH, was introduced.
The release R6 WCDMA UL dedicated physical channel (DPCH) supports up to four types of physical control and data channels including:    DPCCH—carries in a minimal configuration pilots mainly for channel characterization, TPC bits for DL power control, and TFCI bits for transport format coding.    HS-DPCCH—carries ACK/NAK for the L2 HARQ process in the DL HSDPA and channel quality information (CQI) for the DL HSDPA air interface scheduling.    E-DPCCH—carries mainly E-TFCI for E UL transport format information but also other L2 control data for the enhanced UL channels.    DPDCH—carries the L1 data to be demultiplexed into transport channel(s). Multi-code in R6 means that up to 4 DPDCHs can be set up.
The timings of the new physical channels and of the modified DPDCH for the E-DCH are illustrated in FIGS. 4 and 5. FIG. 4 shows WCDMA R6 DPCH subchannel components for a 2 ms Transmission Time Interval (TTI) E-DCH: DPCCH, DPDCH, and E-DPCCH. As with the format shown in FIG. 3, the transport format shown in FIG. 4 suffers from the drawback that the spreading factor used for the DPDCHs carried on the E-DPCCH is not explicitly available until the entire E-DPCCH subframe has been received and demodulated.
FIG. 5 shows WCDMA R6 DPCH subchannel components for a 10 ms TTI E-DCH: DPCCH, DPDCH, and E-DPCCH. The timing of the HS-DPCCH is not illustrated since it is not relevant to the despreading of the DPDCHs. The transport format used for the DPDCHs is mapped to a 2 ms E-DPCCH subframe and repeatedly transmitted 5 times. For air interface efficiency reasons, the E-DPCCH energy required for successful TFCI detection is spread out over all 5 subframes. Thus, not until all five subframes have been soft-combined can the TFCI be decoded from the E-DPCCH with a reasonably high probability of correct decoding. Thus again, the problem is that the transport format which includes the spreading factor SFactual. used for the DPDCHs is not explicitly available until the whole 10 ms frame has been received.
This problem of the transport format used for the DPDCHs not being explicitly available until the whole frame has been received prevents use of the straight-forward Rake receiver illustrated in FIG. 2 because the accumulation length in the integrate-and-dump units is not available until the whole frame has been received. In other words, the accumulation length should be set to the actually used spreading factor, SFactual. One way to address this problem is to perform an initial pre-despreading operation followed by a final despreading operation to convert the direct sequence spread data signal from a broadband chip sequence (chip rate) to a narrowband BPSK-modulated DPDCH data stream (bit rate). So the despreading of the DPDCH data occurs in two steps: (1) pre-despread the composite data signal as it arrives with a pre-configured spreading factor SFpre, and then (2) finally despread the data signal using a final spreading factor SFfinal, which is derived from the SFactual extracted from the TFCI for that data frame using the following relation:SFactual=SFpre*SFfinal 
Pre-despreading without knowledge of the spreading factor is possible in WCDMA due to the construction of the OVSF channelization codes used to separate the physical channels in UL. When viewed over a whole frame or subframe, a particular DPDCH channelization code is actually the same chip sequence for all spreading factors between 4 and 256. However, this is not valid for the SF2 code for E-DCH where multi-code is allowed.
In any event, the actual spreading factor for the frame, SFactual, is not known until after TFCI-decoding for the entire frame is complete. As the equation above indicates, the value of the final spreading factor for the frame SFfinal depends on the actual spreading factor for the frame, SFactual. Consequently, the predespread data needs to be buffered until it can be finally despread. This buffering has disadvantages.
To better understand those disadvantages, reference is made to a receive processor 40 shown in FIG. 6. An oversampled, UE composite signal (includes signals from multiple UEs) from a base station or other receiver antenna is pre-despread in N RAKE finger processing units 41a, 41b, ..., 41n. As explained for FIG. 2, in each pre-despreader finger 41, the composite signal is first delayed for an appropriate time corresponding to the rake finger's channel tap delay, zτ−T, downsampled to the chip-rate (1 ×OS), and then multiplied by the UE's scrambling code cscr and channelization codes cch in a multiplier 43 to extract a UE-specific signal from the composite signal, and the output of the multiplier 43 is integrated in an integrate and dump block 44. The output from each finger is stored in a pre-despread Rake finger delay buffer 45 which delays the one UE's data a sufficient time to allow functions like channel estimation to be determined before passing on the data. A Rake demodulator 46 like that shown in FIG. 2 receives the delayed data along with channel estimates from a channel estimator 55 and performs channel compensation and maximum ratio combining of all the N rake fingers' pre-despread DPDCH symbols corresponding to one UE from the buffer 45 to generate pre-despread data for the one UE which is then stored in a pre-despread, first-in-first-out (FIFO) buffer 48.
For the one UE, the control information for the DPDCH data frame is processed in a parallel path to extract the transport format information that includes among other things the appropriate spreading factor for the DPDCH data frame. A control channel (e.g., DPCCH/E-DPCCH) despreader 54 and demodulator 56 despreads the wideband composite chip stream corresponding to the DPDCH data frame into a narrowband bit stream using the one UE's scrambling code and a control channelization code different than that used to pre-despread the UE composite signal. The TFCI for the UE for this frame is extracted from the bit stream in block 57 and decoded in order to determine the final spreading factor SFfinal used for that frame. A final despreader in the form of another integrate and dump block 50 uses the final spreading factor SFfinal to finally despread the pre-despread data stored in the pre-despread FIFO buffer 48. The finally despread data bits for the one UE, which correspond to “soft” symbol information, are then decoded in decoder 58 into actual data received from the one UE, which are sent on for further processing and transmission to the RNC.
This two-stage despreading is possible because the spreading (channelization) codes cch for the DPDCH in the 3GPP standard were carefully chosen. The spreading factor SFpre is predefined for a certain radio access bearer and corresponds to the lowest allowed spreading factor for the radio access bearer. When the pre-despread spreading factor SFpre and actual spreading factor SFactual are close, the amount of extra, unnecessary data stored in the pre-despreading buffer need not be that large. But there are situations where there is a significant difference between the pre-determined spreading factor SFpre and the actual spreading factor SFactual. In those situations, the amount of unnecessarily despread data to be buffered can be significant.
The buffering problem is even more troublesome with the introduction of an enhanced dedicated channel (E-DCH) in 3GPP release R6. An E-DCH can utilize between 1 and 4 DPDCHs. Table 1 below illustrates various E-DCH options.
TABLE 1Simplified transport formats (TFs) - only SF variation considered.Spreading factor and #Instantaneousmulti-codes forDPDCH BW“TF”DPDCH(kbps)0no data01SF256152SF128303SF64604SF321205SF162406SF84807SF496082× SF4192092× SF23840102× SF2 + 2× SF45760
The set of allowed transport formats (TFs) for the UE is decided on two levels. The first level is set by the RNC when configuring (and reconfiguring) the UE's E-DCH. At that time, the RNC chooses a “superset” of TFs, e.g., 0 to 7 in Table 1, to maximize the UE's instantaneous DPDCH throughput, which in this example case, is a maximize of 960 kbps for SF4. From this superset, an enhanced uplink scheduler in the base station chooses a subset of TFs, and that subset is communicated to the UE in a TF grant. This TF grant can be updated regularly by the enhanced uplink scheduler. The UE then chooses a TF for transmitting each subframe from its granted TF set, depending on the amount of data it currently has queued to transmit uplink. Thus, according to the current TF grant, the UE data frame transmission scheduler may well use a lower TF than the maximum.
But in a worst case scenario, the UL despreader and demodulator 40 in the base station does not receive any information from the UE's uplink scheduler regarding the granted TFs for different UEs. It must therefore allocate despreader and buffering resources according to the RNC-configured maximum transport format, i.e., a worst case where the base station would need a buffer sized for a maximum number of possible UE's that could at one time be communicating with the base station. This is unfortunate because configuring resources assuming a worst case buffering scenario requires very large and costly buffers.
Another problem with such pre-despreading relates to Orthogonal Variable Spreading Factor (OVSF) technique used in 3GPP to generate spreading/channelization codes. The OVSF base code for the channelization code cch is different for a spreading factor SF2 and a spreading factor SF4 for DPDCHs. In practice, this means that for a UE granted a transport format TF=9 (2×SF2), pre-despreaders for both the SF2 and the SF4 codes must be setup because the UE may transmit on either or both of the SF2 and SF4 DPDCH(s). This situation results in a potential 50% larger buffering need which means that suitably sized larger buffers must be provided.